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DEFINITIONS

Alchemy
Database containing scanned Institute of Hydrology gauging station records.

Annual Exceedance Probability ( AEP)
The probability of a flood of given magnitude occurring or being exceeded in any one year.

Auto FEH A method produced by CEH Wallingford (Centre for Ecology and Hydrology, Wallingford) for flood frequency estimation for all rivers in the UK. The Auto FEH flows have been used as the basis for the Indicative River and Coastal Flood Maps (Scotland) and are quoted in this report to allow ready comparison of flood outlines generated using the different methodologies.

Bank locations
When left and right banks are mentioned in the report, this assumes the reader is looking down the channel in the direction of flow.

Catchment area
The area of land from which all surface run-off will collect and contribute to the flow of a specific river.

Deviation from Scheme
Omissions or alterations since construction of the scheme that were not formally part of the flood prevention scheme as recorded in the flood prevention scheme description of works.

Flood Estimation Handbook ( FEH)
The Flood Estimation Handbook was produced by the Institute of Hydrology (now CEH Wallingford) to develop and implement generalised procedures for rainfall and flood frequency estimation in the UK.

Flood Studies Report ( FSR)
Flood estimation methods for river catchments in the UK published in 1975 by the National Environmental Research Council and updated by Supplementary Reports between 1978-1988. The FSR was superseded in 1999 by the Flood Estimation Handbook.

Freeboard
Freeboard is an allowance that takes into account the physical processes that affect the defence level that have not been allowed for in the design water level, together with a safety margin to allow for uncertainties in the prediction of the design water level.

GISmo GISmo provides access to a comprehensive Geographical Information System via Pocket PCs. This provides access to mapping information outside the office for use in data verification and analysis.

HiFlows- UK HiFlows- UK provides flood peak data and station information, at around 1000 river flow gauging stations throughout the UK, for use with the statistical flood estimation methods set out in the Flood Estimation Handbook. The data are provided as both annual maxima and peaks-over-threshold.

Hydrograph
A graphical representation of how a catchment area responds to a period of rainfall or snowmelt. It usually plots river flow against time.

InfoWorks CS/ RS InfoWorks CS and RS represent two software packages from a suite of products produced by Wallingford Software. InfoWorks CS simulates the performance of urban drainage networks during rainfall and is most suited to piped or culverted systems. InfoWorks RS simulates the flood response of largely open channel river systems.

JFLOW 2d raster based hydraulic routing model based on diffusion wave approximation. The model has been used widely for catchment wide flood mapping, breach analysis and overflow modelling.

Joint Probability Analysis
An expression for the study of joint probability, involving the assessment of dependence and prediction of extreme conditions from the distribution and extremes of two related variables.

Manning's 'n'
A coefficient of roughness, used in the Manning's formula to determine the quantity or depth of flow.

Mean High Water Springs
The average of high water levels occurring at the time of spring tides.

Overland flow
Process by which water moves down-slope over the earth's surface in unconfined channels under the influence of gravity.

PAG Project Appraisal Guidelines - published by Defra. A series of 5 documents providing guidance on the development of flood alleviation schemes. For example, PAG 3 relates to cost benefit appraisals of proposed schemes.

Pooling Analysis
When flood records are too short or absent to allow reliable estimation for long return period floods, data from a group of similar catchments can be pooled to extent the length of flood record. The pooling group is the selected collection of gauged stations with similar catchment characteristics to the subject site.

Return Period
The return period is defined as the average elapsed time between events of a given magnitude. This information may also be expressed as the water flow or level which has a particular return period of flooding, for example, 100 years. The inverse of the return period is the statistical probability of an event occurring or being exceeded in any individual year.

'Second Generation' Flood Maps
The SEPA Indicative River and Coastal Flood Map (Scotland). Which will show areas of risk from fluvial and tidal flooding and are intended primarily for planning purposes. In common with practice elsewhere in the UK, they will not show the reduction in flood risk resulting from the construction of flood defences or other flood risk management measures. The maps will replace the 'First Generation' flood maps produced by the Institute of Hydrology in 1996.

Sensitivity analysis
Analysis of the effects on estimated water levels or flows of varying the values of important model parameters.

SOP The standard of protection is the standard (expressed as a return period) that the defences will withstand with a high degree of certainty. The standard of protection is determined for existing schemes by deducting a freeboard from the existing defence level.

Steady/Unsteady Modelling
A steady model is one in which the discharge may differ from point to point but does not change with time. An unsteady model is one in which the discharge varies with time (sometimes known as hydrodynamic modelling).

URBEXT An index of fractional urban extent, based on the Land Cover Map of Great Britain 1990 (produced by CEH Wallingford) and used in the Flood Estimation Handbook. This value has been updated using the digitised urban area from Ordnance Survey maps together with an adjustment factor to reduce the area. This factor is required as the symbolisation of built up areas on the OS 1:10,000 maps tends to exaggerate urban areas.

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